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Article From:https://www.cnblogs.com/coder-2017/p/9123140.html

SSThe frame model is the basis of the layout of a web page – each element is represented as a rectangular box, the content, the inner margin, the boundary, and the outer distance of the frame, which are built on a layer of onions. When the browser renderings the page layout, it will calculate the contents of each box.What style to use, how big the onion layer is, and where the box is relative to the other boxes. Before you understand how to create CSS layout, you need to understand the box model.